Just Do Step by Step:-
Step 1. Connect the iOS device which you want to share through Hotspot.
Step 2. Open app store in the friends iOS device and log in with you apple ID.
Step 3. Go to purchased section and click on cloud button of respective app you want to share.
Step 4. Bingo! You share the app without need to purchases it again.
Step 5. Don't worry about that 5 iOS devices limit because in this step this not affected.
1. Your friends can't update it till you again initiate it. But it's need to activate you iTunes account to that device.
So better I advice you delete the app from you friends device and download it again as mentioned above.
2. Your friends must not select the apps in the his iTunes application in his pc/Mac during updating as it will need to activate iTunes account so better not to do that.
Just deselect in the iTunes sync option.
Otherwise it can't sync and removed from your iOS device.
If you face any problem comment below and required any specific hack let me know trough comments.